Blink comparator
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A blink comparator was a viewing apparatus used by astronomers to find differences between photographic plates taken of the same area of the sky at different times. It was also sometimes known as a blink microscope.
It permitted rapidly switching from viewing one plate to viewing the other, "blinking" back and forth between the two views. In plates taken a few days apart, rapidly moving objects such as asteroids and comets would stand out, because they would appear to be jumping back and forth between two positions, while all the other fixed stars stood still. Plates taken at longer intervals could be used to detect stars with large proper motion, or variable stars, or to distinguish binary stars from optical doubles.
The most notable body to be found using this technique was Pluto, discovered by Clyde Tombaugh in 1930.
The Projection Blink Comparator (PROBLICOM), invented by amateur astronomer Ben Mayer, was a low-cost version of the professional tool. It consisted of two slide projectors with a rotating occulting disk that alternately blocked the images from the projectors. This tool allowed amateur astronomers to contribute to some phases of serious research.
In modern times, CCDs have largely replaced photographic plates and astronomical images are stored digitally on computer. The blinking technique can easily be performed on a computer screen rather than with a physical blink comparator apparatus as before. For example, a recently proposed method allows a radiologist to compare new and old digital medical images (MRI, CT, xrays, etc.) with greater accuracy and efficiency using a digital blink comparator method.
However, the blinking technique is less used than before because image differencing algorithms are often used to detect moving objects more effectively than human eyes are capable of. Or, to measure the precise position of a known object whose direction and rate of motion are known, a "track and stack" technique is used in which multiple images are superimposed by software in such a way that the moving object is fixed in place; the moving object then stands out as a dot among the star trails. This is particularly effective in cases where the moving object is very faint and superimposing multiple images of it permits it to be seen better.
